Optically isotrophic polycarbonate films and a method for the production thereof

ABSTRACT

Injection-molding materials comprised of polycarbonate can be processed into optically isotropic films in a chill roll extrusion method. The optically isotropic films for protecting data carriers (CD-ROMS) from becoming scratched, or they are used as supporting materials for the information layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.09/744,970, filed on Feb. 14, 2001, now allowed as U.S. Pat. No.6,613,264, which is a National Stage (371) of PCT/EP99/05996, filed Aug.16, 1999, which claims priority to DE 198 36 800.3, filed on Aug. 14,1998.

FIELD OF THE INVENTION

The invention relates to polycarbonate films with a high level ofpurity, that are high-gloss, optically isotropic on both sides, as wellas to an economically efficient extrusion process for their production,and the use of the films according to the invention as cover films foroptical data media, such as compact disks or DVD disks, for example.

The films according to the invention can also serve as carrier materialsfor the layer that carries the data, and in this way, extremely thinoptical data media can be produced.

STATE OF THE ART

The demands made for sheets and films made of plastic, so that they aresuitable for producing or covering optically readable data memory media,are generally known, see J. Hennig, Polymere als Substrate für optischePlattenspeicher [Polymers as substrates for optical disk memory media],Angew. Makromolekulare Chemie [Applied Macromolecular Chemistry], Volume145/146, 1986 (page 391-409). Furthermore, the sheets and films are notallowed to contain any foreign particles with a size of more than 10micrometers, and no bubbles are allowed to form during processing. Thedouble refraction resulting from shaping should be as low as possible.

Optical data media in the form of compact disks have been extensivelyproduced using the injection-molding process, up to the present. Otherprocesses that can also be used to produce optical data media with alarger size are based on plain sheets of plastic, which are subsequentlyprovided with the layer that carries the data.

EP 461 485 (Röhm GmbH) describes an extruded plain sheet or film made ofplastic and its production process. The polycarbonate melt is pressedout of a broad-slit die or an extruder at approximately 290° C., andcalendered in a roller frame consisting of a high-gloss steel roller anda padded blanket roller. In this way, a polycarbonate film that ishigh-gloss on one side and matt on the other, with a thickness of 450μm, is obtained.

EP 351 886 (Bayer AG) describes a casting process for the production ofpolycarbonate films. The polycarbonate, with an average molecular weightof 98,000, is dissolved in methylene chloride, and the solution isapplied to a slowly rotating, heated, and polished cylinder, using aductor device. In this way, a clear, transparent film with a thicknessof 200 μm is obtained. A high degree of double refraction results, witha layer difference of ΔG=74 nm. The polycarbonate films obtained in thisway, which are not free of double refraction, are clamped in place andheated for 5 to 60 seconds, using a quartz heat emitter. This results ina reduction of the undesirable double refraction to values that are nolonger a problem.

JP 07 126375 (Teijin Kasei Ltd.) describes the production of a film madeof polycarbonate, with low double refraction. The low specific doublerefraction of the polycarbonate film is achieved by using a protectivelayer made of polyolefins.

CRITICISM OF THE STATE OF THE ART

Conventional methods for the production of thin, optically isotropicpolycarbonate films function either on the basis of costly castingprocesses, in which complicated apparatus measures must be provided toprevent the emission of toxic organic solvents, or on the basis ofcomplicated extrusion processes, in which either a polycarbonate filmthat is matte on one side, or an optically anisotropic polycarbonatefilm is produced in a first step.

These optically anisotropic extrusion films as last mentioned must beconverted into optically isotropic extrusion films that are high-glosson both sides, in subsequent steps. A two-stage extrusion process isextremely costly. The films are not provided with optimum surfaceproperties, although they are optically somewhat isotropic.

Task

The invention was therefore based on the task of producing opticallyisotropic polycarbonate films that are high-gloss on both sides, whileavoiding the economic disadvantages and the disadvantages of productiontechnology that have been described for the state of the art. The filmsare supposed to be optically and mechanically isotropic, have a highlevel of surface quality, and possess the greatest possible opticalpurity.

Solution

Surprisingly, optically high-quality polycarbonate extrusion films in athickness range <200 μm, preferably 15 to 150, especially preferably 30to 100, particularly especially 60 to 90 μm, can be produced using thechill-roll process, with the profile of properties as described. Toachieve the high level of optical purity, a polycarbonateinjection-molding mass with a relatively low molecular weight ispreferably used, such as that used for the production ofinjection-molded optical data media in large amounts.

The molecular weight M_(w) of the polycarbonate molding mass used liesbetween 10,000 and 40,000, preferably between approximately 15,000 andapproximately 20,000 (injection-molding mass). The injection-moldingmass not intended for extrusion use, in particular, can surprisingly beextruded using the chill-roll process, at a high level of economicefficiency and while avoiding toxic solvents, to produce films with therequired profile of properties (see FIG. 1).

IMPLEMENTATION OF THE INVENTION

A significant element for the invention is the use of the melt-castingprocess, also called chill-roll process. In this process, the melt filmthat exits from the die is applied to a cooling roller (chill roll) andcooled as this happens. This makes it possible to produce extremely thinfilms (down to 15 μm) with a low level of optical anisotropism (almostisotropic films).

Almost isotropic is understood to mean that a beam of light penetratingthe film medium undergoes only a deflection that is so low that it canbe ignored, e.g. a layer difference of at most 50 nm, preferably at most35 nm, especially preferably at most 25 nm.

The processing temperature for the polycarbonate molding mass liesbetween 210 and 260, preferably 220 to 240° C.

In order to avoid crystallite deposits in the die, it is advantageous tostart the extrusion system at a processing temperature of 250 to 260° C.To achieve the lowest possible degree of gel body formation, theprocessing temperature can be successively reduced to 220 to 240° C.after the start-up phase of about 10 minutes to about 1 hour.

To avoid die lines or extrusion stripes on the extruded film, it isrecommended to polish the internal extrusion die surface, where it isadvantageous if this surface is chrome-plated, particularly the lipregion. For the extrusion die lip region, the peak-to-valley heightR_(A) according to DIN 4768 should be 0.025 to 0.002, preferably 0.015to 0.002, especially preferably 0.01 to 0.002. It is advantageous if theroughness of the distribution channel is at most 0.1.

Another improvement in quality, particularly the avoidance of die linesor extrusion stripes, of the extruded film can be achieved if theinternal surface of the extrusion die is provided with an agent thatrepels polycarbonate melt. This can be done by coating the cleanedinternal die surface with such an agent, for example silicone oil. Theagent should not increase the peak-to-valley height as a whole, or notincrease it significantly, but preferably it should reduce this height.

Another factor that can influence the quality of the extrudedpolycarbonate film is slight contamination of the polycarbonate melt. Itis therefore advantageous to install a melt filter between the extrusioncylinder and the extrusion die. The mesh size of the filter insertshould be 5 to 50 μm.

Another measure that can contribute to high-quality polycarbonate filmsis mixing a lubricant into the molding mass formulation. Usual amountsare between 0.01 and 1 wt.- % with reference to the molding mass.Examples for suitable lubricants are partially oxidized polyethylene,pentaerythrite stearate, or C₁₀ to C₂₀ fatty acid esters.

A high level of optical purity as well as a high level of opticalisotropy are the significant basic requirements for the use of films inthe stated apparatus.

Using the process according to the invention, it is possible to producepolycarbonate films with high optical quality. High optical quality isparticularly understood to mean: A low level of double refraction, ahigh level of transmission, slight formation of extrusion stripes or dielines (which can be measured by a slight deflection of linear polarizedlight), as well as a low number of gel bodies per area unit.

Preferably, the film according to the invention does not have adeflection of linear polarized light greater than 2 angle minutes in anyregion of the surface.

The measurement is taken by recording the angle of deflection that alaser beam experiences as it penetrates the film. For this purpose, theso-called laser deflection method is used. The angle of deviation that alaser beam of 632,8 nm experiences when passing vertically through thefilm, perpendicular to the extrusion direction, is measured over theentire extrusion width, using a displacement table that is operated bymeans of a stepper motor. The transmitted beam then passes through atelescope, in order to amplify the angle of deflection, and hits aposition-sensitive digital optical sensor. This sensor determines thelocation of the center of the point of light that impacts on it, andoutputs it in the form of x and y coordinates, via a serial interface.Only the y component is relevant for an assessment of die lines.

Possible uses of the films according to the invention are asscratch-protection films for optical data media, as carrier materialsfor optical data media, as base materials for the production of overlayfilms for displays and screens.

EXAMPLES Example 1

Production of the film according to the invention by means of chill-rollextrusion of a polycarbonate molding mass for optical injection-moldingapplications.

The melt, produced by means of a single-screw or twin-screw extruder (toensure constancy of the melt flow, a melt pump can optionally be used),is passed to the chill-roll roller (peak-to-valley height R_(A)0.002-0.006, R_(T)=0.02-0.004, measured according to DIN 4768), via anautomatically regulated die designed for film extrusion. Thepeak-to-valley height R_(A) of the extrusion die lip region is 0.02. Theinternal surface of the die was provided with silicone oil, to make itrepel polycarbonate melt. The temperature of the melt flow is 235° C.±5°C. The melt film comes to rest tangentially against the roller surface,and surrounds the roller by about 180° C. After having passed aroundadditional rollers, the thickness of the film web is determined using acontact-free measurement system that is arranged on a traverse, and themelt distribution of the die is regulated over the width, using anexpansion bolt system, based on the electronically processed data. Therelativized films possess a high level of optical and also mechanicalisotropism. The latter is particularly important for the processingbehavior, e.g. punching to CD size, since PC injection-molding massesare clearly more brittle than PC extrusion masses, because of their lowmolecular weight.

Comparison Example 1

Polycarbonate film produced using the planishing process, thickness 0.13mm.

The melt, produced by means of a single-screw or twin-screw extruder (toensure constancy of the melt flow, a melt pump can optionally be used),is passed to the planishing rollers via a die designed for filmextrusion. The melt is sized in a defined roller nip and planished andcooled by the surface of the tempered rollers, which are polished to ahigh-gloss mirror finish (peak-to-valley height R_(A) 0.002-0.006,R_(T)=0.02-0.04, measured according to DIN 4768), via an automaticallyregulated die designed for film extrusion. In this connection, thegeometric shape of one or both rollers, deviating from a cylinder shape,is ground to a bomb-type shape. The bomb curvature is 0.1 to 0.2 mm,with reference to the diameter of the roller. The bomb curvature is ofdecisive significance for a uniform thickness distribution over thewidth of the film web.

Because of the high roller nip forces that are typical for this process,the polymer molecules are stretched lengthwise in the extrusiondirection. The resulting orientation leads to marked opticalanisotropism, which is clearly too high.

Comparison Example 2

Production of a cast polycarbonate film with a thickness of 0.08 mmtakes place according to the following process:

Polycarbonate is dissolved in a mixing container equipped with astirring mechanism, in a solvent mixture of acetone and methylenechloride.

Gross contaminants are subsequently removed from the polycarbonatesolution, using a 2 μm membrane filter press. The polycarbonate solutionis subsequently processed in a casting machine that is composed of asteel strip with a length of 48 mm, driven by two drums with a diameterof 3 m, to produce a film with a thickness of 80 μm. The solvent, whichevaporates off during the subsequent drying process, is returned to theprocess.

The resulting PC cast films do possess sufficiently low opticalanisotropism, but they are not free of disruptive optical effects, suchas gel particles and stripes.

Production of cast films from organic solution is a costly process,which also sets great demands with regard to safety technology. Theresulting films are not free of the highly toxic solvent, and thereforethey are problematic, from a toxicological point of view.

The large-area steel strip is clearly more difficult to produce andmaintain in a high-quality optical condition than the clearly lesserarea of the chill-roll roller of the melt-casting process according tothe invention. This results in an impermissibly high number of stripeson the film surface. In spite of the use of 2 μm membrane filterpresses, it is not possible to avoid gel-body-like aggregates thatreduce the laser signal intensity. In contrast to melt processing inextruder systems, there is no shear field applied in “solution casting,”and therefore the portions of the polycarbonate with a high molecularweight, which are difficult to break up, aggregate because of thepossibility of “micro-Brownian movement,” i.e. they strive towards thecondition of the lowest possible interfacial energy.

The gel bodies and stripes reduce the laser signal intensity to animpermissibly low level, resulting in a significant number of errormessages when playing CD systems protected in this way.

Reference Symbols 10 melt film 15 film formed from the melt film 20chill-roll roller 30 die

Property Shrinkage Tensile test ISO 527-3 190° C./30 min. Layer DoubleElongation to tear [%] Haze [%] difference refraction [%] ExampleMD*/TD* ASTM-1003 [nm] Δn MD*/TD* Film according to the invention <1/<10.5  31 4 · 10⁻⁴  39/35 Chill-roll extrusion of PC molding mass forinjection- molding of optical data media, thickness: 0.08 mm ComparisonExample 1 31/−7 0.3 360 3 · 10⁻³ 145/99 PC film extrusion molding mass,produced using the planishing process, thickness: 0.13 mm ComparisonExample 2 3 1.2  29 4 · 10⁻⁴ not measured PC film, cast from organicsolution (trade name Pokalon), thickness: 0.08 mm *MD: machine direction*TD: traverse direction Comment: “Shrinkage” distinguishes the inventionfrom Comparison Example 1. Haze distinguishes the invention fromComparison Example 2. Double refraction and layer difference distinguishthe invention from Comparison Example 1. Elongation to teardistinguishes the invention from Comparison Example 1.

1. A polycarbonate film obtained by a process comprising i) extruding apolycarbonate injection-molding mass through an extrusion die to form apolycarbonate film; ii) passing said polycarbonate film to a chillroller; and iii) cooling said polycarbonate film, wherein saidpolycarbonate film has a thickness of less than 200 μm and wherein saidpolycarbonate film does not exhibit, in any region of the surface, adeflection of linearly polarized light of greater than 2 minutes ofangle, where the angle of deflection is measured by a laser deflectionmethod at 632.8 nm.
 2. A scratch protection film for optical data mediacomprising the polycarbonate film of claim
 1. 3. A carrier material foroptical media comprising the polycarbonate film of claim
 1. 4. A basematerial for the production of overlay films for displays and screenscomprising the polycarbonate film of claim
 1. 5. The polycarbonate filmof claim 1, wherein said thickness is in the range of from 15 to 150 μm.6. The polycarbonate film of claim 1, wherein said thickness is in therange of from 30 to 100 μm.
 7. The polycarbonate film of claim 1,wherein said thickness is in the range of from 60 to 90 μm.